Re: [PATCH v2 2/3] Add zero-copy-copied migration stat

[Dr. David Alan Gilbert]

* Daniel P. Berrangé (berrange@redhat.com) wrote:
> On Mon, Jul 04, 2022 at 08:18:54AM +0200, Markus Armbruster wrote:
> > Leonardo Bras <leobras@redhat.com> writes:
> > 
> > > Signed-off-by: Leonardo Bras <leobras@redhat.com>
> > > ---
> > >  qapi/migration.json   | 5 ++++-
> > >  migration/migration.c | 1 +
> > >  monitor/hmp-cmds.c    | 4 ++++
> > >  3 files changed, 9 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/qapi/migration.json b/qapi/migration.json
> > > index 7102e474a6..925f009868 100644
> > > --- a/qapi/migration.json
> > > +++ b/qapi/migration.json
> > > @@ -55,6 +55,9 @@
> > >  # @postcopy-bytes: The number of bytes sent during the post-copy phase
> > >  #                  (since 7.0).
> > >  #
> > > +# @zero-copy-copied: The number of zero-copy flushes that reported data 
> > > sent
> > > +#                    using zero-copy that ended up being copied. (since 
> > > 7.2)
> > 
> > The description feels awkward.  What's a "zero-copy flush", and why
> > should the user care?  I figure what users care about is the number of
> > all-zero pages we had to "copy", i.e. send the bulky way.  Is this what
> > @zero-copy-copied reports?
> 
> MigrationCapability field @zero-copy-send instructs QEMU to try to
> avoid copying data between userspace and kernel space when transmitting
> RAM region.
> 
> Even if the kernel supports zero copy, it is not guaranteed to happen,
> it is merely a request to try.
> 
> QEMU periodically (once per migration iteration) flushes outstanding
> zero-copy requests and gets an indication back of whether any copies
> took place or not.
> 
> So this counter is a reflection of how many iterations resulted  in
> zero-copy not being fully honoured.
> 
> IOW, ideally this counter will always be zero. If it is non-zero,
> then the magnitude gives a very very very rough guide to what's
> going on. If it is '1' then it was just a transient limitation.
> If it matches the number of migration iterations, then it is a
> more systemic limitation.
> 
> Incidentally, do we report the migration iteration count ? I
> thought we did, but i'm not finding it now that I look.

Yes we do; it's dirty-sync-count

Dave

> 
> With regards,
> Daniel
> -- 
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-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK